The present invention is concerned with plasticized interlayers for laminated glazing units particularly laminates in which one of the layers in contact with the plasticized interlayer is formed from a resin subject to crazing when contacted with organic solvents.
The conventional safety glass used in automobile windshields, security glass and in a variety of architectural applications comprises two sheets of glass with an interlayer of a tough transparent plastic such as a plasticized polyvinyl acetal.
These structures have proved extremely successful for many years but they are not without their drawbacks. Because they comprise two sheets of glass they carry a weight penalty that it is often desirable to avoid, particularly in automobiles where weight savings can be quickly converted into improved gas economy. In addition, where the laminates are used to produce automobile windshields it has been found that in a crash, should any of the passengers be thrown head forward into the windshield with sufficient force, the inboard glass layer of the laminate will shatter. Any subsequent sliding motion against the fragments of shattered glass, which are held in place by the interlayer, leads to severe lacerations of the head even though the head does not penetrate the windshield.
Considerations like the above have led to the proposal that at least one of the glass layers of laminates be replaced by a transparent rigid thermoplastic resin that would be much lighter than glass, would not be so easily shattered and, if broken, would not produce dangerous fragments with sharp cutting edges.
Another very significant use of such units is in the field of security glazing where a layer of a suitable thermoplastic resin can impart substantial toughness when laminated into a glazing unit making it very suitable for security purposes as for example in banks and in prisons without bars.
Among such transparent rigid, thermoplastic resins are the polycarbonates and the polyacrylates such as polymethyl methacrylate. These have excellent clarity, rigidity and hardness and are easily produced in sheet form. Unfortunately, they are somewhat susceptible to crazing and accelerated stress cracking in the presence of many organic compounds -- including compounds commonly used as plasticizers for the polyvinyl acetal interlayer.
The phenomenon of "crazing" referred to above is the development of a network of minute surface cracks in the polycarbonate or polyacrylate sheet that cover the surface and produce a whitening effect that destroys the clarity of the sheet.
It has therefore, been necessary to develop a plasticizer for polyvinyl acetal resins that is adapted for use in interlayers in contact with polycarbonate and polyacrylate sheet. An effective plasticizer for use in a polyvinyl acetal interlayer in contact with such resins must therefore:
(1) plasticize the polyvinyl acetal interlayer effectively; PA1 (2) not "craze" the sheet in contact with the interlayer; PA1 (3) be stable at the temperature at which the interlayer is extruded; PA1 (4) be stable to ultraviolet light; PA1 (5) be stable to action of water; PA1 (6) have low color through the temperature range likely to be encountered; PA1 (7) not impair adhesion of the interlayer to the resin sheet; and PA1 (8) be able to tolerate a small amount of water without giving rise to phase separation in the interlayer (blushing).